Although not a new idea, high pressure fluid hydroforming is now finding increasing use in vehicle frame members, because of its ability to integrally form a hollow beam or the like with a complex shape that would otherwise have to be fabricated from several pieces and/or bent to shape. Typically, a round tubular blank is inserted between a pair of dies which together form an internal cavity that matches the exterior shape desired for the final part. Then the ends of the tubes are plugged and the interior of the tubular blank is highly pressurized, generally with water or hydraulic fluid, to force it out into the shape of the die cavity. Then, the completed part is depressurized and removed.
Often, various holes or openings may be desired in the wall of the final part. These may be drilled or cut after the fact, but it is also well known to incorporate the hole piercing process into the hydroforming process itself, at least when the holes are one sided. That is, when the hole is a single hole through one side only of the tube, and not a two sided hole (pair of aligned holes), such as would be formed by pushing a drill across and through both sides of the tube. There are two basic existing approaches to cutting single holes in the die, which may be termed an active and a passive process. U.S. Pat. No. 3,487,668 issued Jan. 6, 1970 to Fuchs shows the active approach. Once the tube has been pressurized internally, a sharp edged punch is pushed through a guide journal in the die and into the pressurized tube interior. Clearly, the punch must be closely enough contained within its guide journal to prevent excessive fluid leakage. The pressurized fluid inside the tube acts like an interior mandrel or support to keep the punch cutting edge from deforming and extruding the tube material inwardly to too great a degree. Still, some so called "countersinking", a slightly conical edge around the perimeter of the hole, is inevitable. This countersinking may even be desirable in the case where a screw or the like is to be inserted into the hole later, because it will act as a lead in. The slug that is cut out when the punch passes through may be handled in two ways. If the punch edge is not too sharp, then the slug may simply stick to one edge of the hole and be swung inwardly, like a hinge, without failing off into the interior of the tube. Often, this is desired, since it obviates the need to shake or wash the slug out of the tube interior later. If the punch is sharp edged, then the slug will be cut away completely and simply fall into the interior of the tube, from which it is later removed.
A variation of this active punch approach for punching single sided holes is shown in U.S. Pat. No. 4,989,482 issued Feb. 5, 1991 to Mason, some of the disclosure of which is reproduced below in FIGS. 6 and 7. In some sense, it is a solution in search of a problem, since it is claimed in the patent that then existing methods of active hole punching through the wall of an internally pressurized tube tend to leave the slug attached to the edge of the hole. While this is true for dull punches, as indicated above, the Fuchs' patent clearly shows that a sharp punch will clearly shear out a slug, although the slug tends to then fall off into the interior of the tube. The one patent claims to keep the sheared off slug stuck to the end of the punch with a suction cup action. As shown in FIGS. 6 and 7, the punch P has an end face that is machined with a large, shallow, suction cup shaped face C. The center of the shallow curved face C is vented with a small diameter, central vent passage V. The annular edge E on the end of the punch is ninety degrees at its outer diameter, where it intersects the cylindrical outer wall of the punch P, but is effectively dull overall, since it makes a very gradual and shallow transition into the shallow, cup shaped face C. This is deliberate, because what is intended, as shown in FIG. 6, is for the punch P to pierce the pressurized tube T and cut out a slug S (leaving the inevitably countersunk hole rim R) while maintaining a constant, if thin, space between the surface of the curved end face C and the back surface of the slug S. This thin space, in turn, is continually vented to atmosphere to create a claimed negative pressure differential (relative to the pressurized tube interior) that will keep the slug S suctioned onto the end of the punch P. For this space and the suction that exists in the space to be maintained, of course, the slug S cannot deform far enough inwardly to abut the curved face C. The continual suction is described as evening out the shear force so that the slug S is cleanly cut, rather than to be left adhering to the edge of the hole like a lid on the edge of a can. While it is questionable that this suction action is needed simply to cleanly shear off a slug like S (a sharp edged punch will do that much), the ultimate objective is really to flatten out, from the inside, the slightly countersunk hole rim R. This is achieved because the adhered slug S is slightly greater in diameter than the punch edge E, overlapping it and leaving a narrow, but noticeable residual ring of width delta. This wider slug S, since it is sucked onto the end of punch P, can be backed up along with punch P and against the edge rim R, flattening it, so it is claimed.
The method just described is of dubious utility because, if the desired objective is simply to create a flat edged hole without the countersunk rim R, then a preferable method for doing so, in many cases, is the passive method of hole punching. As shown in co assigned U.S. Pat. No. 5,398,533 to Shimanovski et al, a flat edged hole may be cut by letting the internal of the internal fluid blow a hole through the tube wall from the inside out, through a sharp edged female die that abuts the outside of the tube and acts as a template. A plunger acting as a backup support of the tube wall within the die is pulled back when the pressure is sufficient, thereby leaving the tube wall unsupported over the die and allowing the hole to be blown through. The slugs are then ejected through a special mechanism. This patent shows an improvement of a far older apparatus for passive cutting disclosed in U.S. Pat. No. 3,495,486. There, a tube is internally pressurized for the sole purpose of passively cutting a hole with a plunger backed female die button, although some of the structural features of the apparatus could be applied to in die piercing combined with hydroforming. There is a recognition in the '486 patent that a cut slug can be deformed and wedged up tightly into the concave end of a sharp edged plunger, as shown in FIG. 8 of the patent, where a slug 156 is bowed into the sharp edged concave face of a plunger 138 and inside of a concentric hole in a sliding sleeve 114. The sleeve 114 then has to be pulled in order to eject the slug 156. In general, passive punching can be used where the hole is large enough to in turn present enough unsupported area across a female die button to be blown out. It also presents special sealing problems in that excessive loss of the highly pressurized fluid has to be prevented. Active punching is less sensitive to leakage and pressure losses, since the male punch is quickly and forcibly pushed through the tube, and pressure loss will not jeopardize its cutting action. Neither method and apparatus could be easily applied to punching two aligned holes in a tube, however, unless both dies contained enough room to have two identical apparatuses (punches or plunger backed die buttons) arrayed in an opposed relationship. Neither method used alone could provide two aligned holes in which one hole had a countersunk rim, and the other was flat edged, if that was the ultimate goal.